The website of the St. Louis Genealogy Society (SLGS) is a must for anyone doing research in city. The 'Irish in St. Louis' section of the website has three very useful listings. First, you will find a list of all the Roman Catholic parishes in the city and their year of foundation. The introduction helpfully points out that researchers should "compare the church address and your ancestor’s address on a St. Louis map" to determine which Church a person of interest may have worshipped at. Thankfully, the website has a map section.
I have previously written about Repeal Associations in relation to Terrence Punch's work in Prince Edward Island, Canada newspapers. A meeting of the Repeal Association of the Friends of Ireland took place in St. Louis on 10 May, 1842. The Missouri Republican newspaper reported on the meeting and an article listed 172 men and where the Irish-born, which comprised the vast majority, were from in Ireland. The SLGS has reproduced the list and helpfully added information for many of the men from other record sets.
There is also a collection of notices from local newspapers concerning Irish immigrants and their descendants. The vast majority are death notices, with some information wanted adverts also included. Some included Irish places of origin.
In conjunction with these useful resources, also check out my blog post about Ward 2 of St. Louis in the 1860 U.S. Federal Census. Irish places of origin were recorded for more than 300 people.